Magnetorheological (MR) devices of the “rotary-acting” or “linear-acting” variety such as linear dampers, rotary brakes and rotary clutches employ magnetorheological fluids within the working gap that are comprised of magneto-soft particles or as such particles dispersed within a liquid carrier and referred to as MR fluids. The higher the applied magnetic field strength, the higher the damping or resistive force or torque needed to overcome the particle structure.
MR fluid devices are disclosed in U.S. Pat. No. 5,816,372 entitled “Magnetorheological Fluid Devices And Process Of Controlling Force In Exercise Equipment Utilizing Same”; U.S. Pat. No. 5,711,746 entitled “Portable Controllable Fluid Rehabilitation Devices”; U.S. Pat. No. 5,842,547 entitled “Controllable Brake”; U.S. Pat No. 5,878,871 entitled “Controllable Vibration Apparatus” and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention.
The present invention as a device includes a housing or chamber that contains the magnetically controllable fluid disclosed herein below, with a movable member, a piston or rotor, mounted for movement through the fluid in the housing. The housing and the movable member both include a magnetically permeable pole piece. A magnetic field generator produces a magnetic field across both pole pieces for directing the magnetic flux to desired regions of the controllable fluid. Such devices require precisely toleranced components, expensive seals, expensive bearings, and a relatively small volume of magnetically controllable fluid. MR devices provide as currently designed are comparatively expensive to manufacture. There is a continuing need for reducing the cost of controllable MR devices for providing variable forces and/or torques.
Conventional MR fluids containing magnetically active fine particles generally on the order of 1–100 μm average diameter employ conventional iron particles manufactured by the carbonyl process, whereby particles are grown by precipitation of pentacarbonyl salts. Magnetorheological fluids have been manufactured that employ magnetically active particles manufactured by an atomization method, which is a reductive process of dividing a molten metal stream into small particles. The molten metal stream is delivered into a high pressure, high velocity stream and divided by high shear and turbulence (hereinafter collectively referred to as “atomized particles”).
Due to performance and cost concerns, suitable replacement for expensive carbonyl iron by atomized particles has not been a straightforward substitution. In conventional practice heretofore, atomized particles of a single process stream have been sieved to exclude a significant fraction of 10–20% of particles larger than 74 μm. In other instances, an even larger fraction of 20–30% of a single process yield of atomized particles greater than 45 μm size must be excluded to render the population useful for magnetically controllable devices. Yields below 90% are considered uneconomical.
Attempts have been made to blend atomized particles with carbonyl iron particles to achieve a suitable particle size distribution for use in MR devices. Heretofore, attempts to provide 100% of particles passing through a 74 μm sieve and approaching a Gaussian distribution have been achieved by blending particles from more than one process stream. U.S. Pat. No. 6,027,664 (Lord Corporation) teaches blends of a first population having an average particle diameter 3 to 15 times larger than the second population. Such mixtures are uneconomical in part because yield losses from the atomized process are carried over from classification or sieving. The suitability of any particulate metals for use in MR fluids is in one respect determined by analyzing the degree of deviation from a Gaussian distribution, and can be illustrated by a regression analysis. Mixtures of two different populations heretofore taught in the art also approach a Gaussian distribution but doe not equal the distribution provided by carbonyl iron powders. For example, a 50:50 wt. mixture of carbonyl iron and water-atomized particles available as of the filing date of the '664 patent deviate from a log normal size distribution with an R2 of 0.82. Although technically feasible, the particle blends heretofore available suffer from economic drawbacks. A need therefore exists for particles utilized in MR devices utilizing atomized particles of a single process stream in higher yield of useable particles with improved distribution, which has heretofore not been met. It would be advantageous to provide a MR fluid containing a particle component derived from a single economical process yield having a population of magnetically responsive particles exhibiting a useful size distribution for improving economic factors in controllable devices.